The present invention has for its aim a process and plant for the preparation of concentrated aqueous solutions of alkali metal hypochlorite, for example sodium hypochlorite, having a strength greater than 63 chlorometric degrees.
Commercial Javel extracts are dilute aqueous solutions of sodium hypochlorite having a strength between 30 and 70 chlorometric degrees, normally between 35 and 50 chlorometric degrees.
The strength of an aqueous solution of alkali metal hypochlorite in chlorometric degrees is defined as the amount of liters of gaseous chlorine at 0.degree. C. and 1 bar having the same oxidizing power as one liter of the solution at 20.degree. C.
It is known that commercial Javel extracts can be prepared by reacting chlorine, possibly diluted with air, with aqueous solutions of sodium hydroxide. The resulting extracts of Javel are therefore aqueous solutions of sodium hypochlorite, containing dissolved sodium chloride, and their stability over a period of time is uncertain.
With the object of obtaining commercial Javel extracts of improved stability, it has already been proposed that there should be first produced a concentrated aqueous solution of sodium hypochlorite, having a strength of at least 70 chlorometric degrees, the said solution being then diluted to obtain the strength required for the extracts of Javel of commerce (U.S. Pat. No. 3,287,233 published on Nov. 22, 1966 and assigned to Societe d'electro-chimie, d'electrometallurgie et des acieries electriques d'Ugine).
In the operation of this known process, the production of the concentrated aqueous solution of sodium hypochlorite by reaction of chlorine with the aqueous solution of sodium hydroxide is accompanied by the formation of crystals of sodium chloride which it is advisable to separate.
In general, the separation of the crystals of sodium chloride from the concentrated solution of sodium hypochlorite presents great difficulties because of their fineness.
To remove this disadvantage, it is proposed, in the U.S. patent quoted, to react chlorine and the aqueous solution of sodium hydroxide inside a fluidised bed of sodium chloride crystals, maintained by a rising circulation of an aqueous solution of sodium hypochlorite. In this way a concentrated solution of sodium hypochlorite, largely free from sodium chloride crystals, is obtained at the top of the fluidised bed, while the large crystals of sodium chloride settle out and are recovered from the bottom.
This known process for the production of concentrated aqueous solutions of sodium hypochlorite requires the use of a fluidised bed which requires high capital investment for an expensive plant which is difficult to operate.
In Chemical Abstracts, vol. 92, No. 10, March 1980, page 147, column 2, abstract No. 78993v. (Japanese patent application No. 79118398--Asahi Glass), a process is described for the production of a concentrated aqueous solution of sodium hypochlorite, wherein a dilute solution of sodium hypochlorite and sodium chloride is made to circulate from a mixing chamber where it is mixed with a concentrated solution of sodium hydroxide to a reaction chamber where the resultant mixture is treated with a gas containing chlorine. In the reaction chamber sodium hypochlorite is formed and sodium chloride crystallizes and precipitates. When the solution in the reaction chamber has achieved the selected strength in sodium hypochlorite, the process is stopped and the reaction chamber is drained to extract thereof the solution of sodium hypochlorite and the crystals of sodium chloride.
This known process has the disadvantage that it is not adapted to a continuous production of solutions of sodium hypochlorite with a constant strength.